FIG. 1 depicts an ink jet colour printer on which the main parts are labelled as follows: a fixed structure 41, a scanning carriage 42, an encoder 44 and, by way of example, printheads 40 which may be either monochromatic or colour, and variable in number.
The printer may be a stand-alone product, or be part of a photocopier, of a plotter, of a facsimile machine, of a machine for the reproduction of photographs and the like. The printing is effected on a physical medium 46, normally consisting of a sheet of paper, or a sheet of plastic, fabric or similar.
Also shown in FIG. 1 are the axes of reference:
x axis, horizontal, i.e. parallel to the scanning direction of the carriage 42; y axis, vertical, i.e. parallel to the direction of motion of the medium 46; z axis, perpendicular to the x and y axes, i.e. substantially parallel to the direction of emission of the droplets of ink.
The composition and general mode of operation of a printhead according to the thermal type technology, and of the “top-shooter” type in particular, i.e. those that emit the ink droplets in a direction perpendicular to the actuating assembly, are already widely known in the sector art, and will not therefore be discussed in detail herein, this description instead dwelling more fully on some only of the features of the heads and the manufacturing process, of relevance for the purposes of understanding this invention.
The current technological trend in ink jet printheads is to produce a large number of nozzles per head (≧300), a definition of more than 600 dpi (dpi=dots per inch), a high working frequency (≧10 kHz) and smaller droplets (≦10 pl) than those produced in earlier technologies.
Requirements such as these are especially important in colour printhead manufacture and make it necessary to produce actuators and hydraulic circuits of increasingly smaller dimensions, greater levels of precision, narrow assembly tolerances; they also accentuate the problems created by the different thermal expansion coefficients of the various materials of the head.
Great reliability is also required of the heads, especially when making provision for interchangeable ink tanks: the useful life of these heads, known as semi-fixed refill heads, is in fact close to the printer life time.
Thus the need to develop and produce fully integrated monolithic heads, in which the ink channels, the microelectronics of selection, the resistors and the nozzles are integrated in the wafer.
In Italian patent application No. TO 99 A 000610 “Monolithic printhead and associated manufacturing process” a monolithic ink jet printhead is described, that comprises an actuator 50, illustrated in FIG. 2, which in turn consists of a die 61 and a structure 75, the latter containing two rows of nozzles 56. The die 61, of a semiconductor material (usually Silicon), comprises a microelectronics 62 and soldering pads 77, permitting the electrical connection of the microelectronics 62 to the printer control circuits. Microhydraulics 63 belong partly to the structure 75 and partly to the die 61.
In the technology relating to that patent application, the nozzles 56 have a diameter D of between 10 and 60 μm, while their centres are usually spaced apart by a pitch A of 1/300th or 1/600th of an inch (84.6 μm o 42.3 μm). Generally, though not always, the nozzles 56 are arranged in two rows parallel to the y axis, staggered one from the other by a distance B=A/2, in order to double the resolution of the image in the direction parallel to the y axis; the resolution thus becomes close to 1/600th or 1/1200th of an inch (42.3 μm or 21.2 μm). The x, y and z axes, already defined in FIG. 1, are also shown in FIG. 2.
FIG. 3 shows the section AA, parallel to the plane z-x, and the section BB, parallel to the plane x-y, of the same actuating assembly 50, where the following may be seen:                a plurality of nozzles 56, arranged in two rows parallel to the y axis;        a plurality of chambers 57, arranged in two rows parallel to the y axis;        a groove 45, having its greater dimension parallel to the y axis, and accordingly to the rows of the nozzles 56.        
Enlarged views of the same sections are shown in FIG. 4, which includes the following parts:                the structure 75, made of a layer of, for example, polyamide or epoxy resin, having a thickness preferably between 30 and 50 μm and in turn containing:        one of the nozzles 56 of said plurality;        one of the chambers 57 of said plurality;        ducts 53.        
Also shown in this figure are:                a substrate 140 of Silicon P;        the groove 45, comprising two parallel walls 126;        a lamina 64, in turn made of, as a non-restricting example, the following layers:        a diffused “N-well” layer 36 of Silicon;        an insulating LOCOS layer 35 of SiO2;        a resistor 27 of Tantalum/Aluminium having a thickness of between 800 and 1200 Å;        a layer 34 of polycrystalline Silicon;        an interlayer 33 of BPSG;        an interlayer 32, consisting of a layer of SiO2;        a “second metal” 31;        a layer 30 of Si3N4 and of SiC for protection of the resistors; channels 67; and        a conducting layer 26, consisting of a layer of Tantalum covered by a layer of Gold and divided into segments 26A, indicated by the dashed lines in the figure, which cover entirely the bottom of each chamber 57.        
The microhydraulics 63 of an actuator 50 may now be defined as the whole comprising the nozzles 56, chambers 57, ducts 53 and channels 67, and serves the purpose of bringing the ink 142, contained in the groove 45 and in a tank not shown in the figures, to the nozzles 56.
Another actuator 50 is shown in FIG. 5, but this time sectioned parallel to the z plane according to a section DD which is shown enlarged in FIG. 6. The groove 45 and the lamina 64 are seen sectioned according to their longitudinal direction, i.e. parallel to the y axis. Two feedthrough contacts 123 are visible along this section which produce the electric contact between the conducting layer 26 and the N-well layer 36. In correspondence with each feedthrough contact 123, the insulating layers 30, 32 and 33, and the layer 34 of polycrystalline Silicon are taken out, whereas an N+ contact 37 and a “metal” 25 of Aluminium/Copper are grown. The succession of the layers 26, 25, 27 and 36, all strictly in contact with one another and all made of electrically conducting materials, ensures electrical continuity between the conducting layer 26 and the N-well layer 36.
The process of manufacture of the actuator 50 for said monolithic ink jet printhead will now be described in brief. This process initially comprises the production of a “wafer” 60, as indicated in FIG. 7, consisting of a plurality of dice 61, each of which comprises an area 62′, suitable for accommodating the microelectronics 62, and an area 63′, suitable for accommodating the microhydraulics 63.
In a first part of the process, when all the dice 61 are still joined in the wafer 60, all of the microelectronics 62 are produced and completed and, at the same time, the microhydraulics 63 of each die 61 are partly produced, using the same process steps and the same masks.
In a second part of the process, on each of the dice 61 still joined in the wafer 60, the structures 75 are made and the microhydraulics 63 are completed by means of operations compatible with the first part of the process. At the end of the process the dice 61 are separated by means of a diamond wheel: the whole consisting of a die 61 and a structure 75 thus constitutes the actuator 50 (FIG. 8).
The first and second part of the monolithic head manufacturing process are described in detail in said Italian patent application No. TO 99 A 000610. The summary description that follows, concerning the second part of the process, contains solely the information needed for an understanding of this invention, and refers to the flow diagram of FIG. 9.
In the step 100, the wafer 60 is available as it stands at the end of the first part of the process, completed in the areas of the microelectronics 62, protected by the protective layer 30 of Si3N4 and SiC, upon which the conducting layer 26 is deposited, and ready for the subsequent operations in the areas of the microhydraulics 63.
In the step 101, etching commences of the groove 45 by way of the “dry” type technology called ICP (“Inductively Coupled Plasma”), known to those acquainted with the sector art. The part of the groove 45 made in this stage has only the walls 126, substantially parallel to the plane y-z (FIGS. 4 and 6).
In the step 102, etching of the groove 45 is completed by way of a “wet” type technology using, for example, a bath of KOH (Potassium Hydroxide) or TMAH (Tetrametil Ammonium Hydroxide), as is known to those acquainted with the sector art. Etching of the groove 45 progresses according to geometric planes defined by the crystallographic axes of the silicon, and therefore forms an angle α=54.7°, as illustrated in FIGS. 4 and 6.
The etching is stopped automatically when the N-well layer 36 is reached by means of a method, called “electrochemical etch stop”, known to those acquainted with the sector art.
Following this operation, the groove 45 is delimited by the lamina 64, seen according to section AA in FIG. 4 and section DD in FIG. 6.
In the step 103, by means of the dry etching technology known to those acquainted with the sector art, the channels 67 seen in FIG. 4 are produced, having a diameter preferably between 5 and 20 μm.
In the step 104, electrodeposition of the sacrificial metallic layer 54 is performed.
In the step 105, a structural layer of thickness preferably between 15 and 60 μm and consisting of a negative epoxy or polyamide type photoresist is applied to the upper face of the die 61 which contains the sacrificial layers.
In the step 106, on the structural layer, the nozzles 56 are opened by means of, for instance, laser drilling, and are freed of the photoresist in the areas corresponding to the solder pads 77 and the heads of the dice. In this way, all that remains of the structural layer is the structure 75.
FIG. 10 shows a section CC, parallel to the plane z-x, of the actuator 50 as it appears at this stage of the work.
In the step 107, the structure 75 is hard-baked in order for it to completely polymerize.
In the step 110, the sacrificial layer 54 is removed in an electrolytic process. The cavity left empty by the sacrificial layer 54 accordingly comes to form the ducts 53 and the chamber 57, already illustrated in FIG. 4, the shape of which reflects exactly the sacrificial layer 54.
The technology described from step 104 to step 110 is known to those acquainted with the sector art, and belongs to the technology designated by the abbreviation MEMS/3D (MEMS: Micro Electro Mechanical System).
In the step 111, etching is performed on the protective layer 30 of Si3N4 and SiC in correspondence with the solder pads 77.
In the step 112, the wafer 60 is cut into the single dice 61 using a diamond wheel, not depicted in any of the figures.
Finally in the step 113, the following operations, known to those acquainted with the sector art, are carried out:                soldering of a flat cable on the die 61 via a Tape Automatic Bonding (TAB) process, for the purpose of forming a subassembly;        mounting of the subassembly on the container of the head 40;        filling of the ink 142;        testing of the finished head 40.        
In FIG. 9 the following steps in particular are highlighted by means of bold face characters:
Step 102, wet etching of the oblique walls of the groove 45, with an electrochemical etch stop; step 104, electrodeposition of the sacrificial layer 54; and step 110, electrolytic removal of the sacrificial layer 54.
In correspondence with the steps, operations are carried out in the form of electrochemical processes, during which specific layers belonging to all of the dice 61 of the wafer 60, and where applicable to all the segments into which the dice 61 are divided, must be put at the same electric potential.
According to the known art, this may be done as illustrated schematically in FIG. 11, in which the following can be seen:                a wafer 60, represented in section, immersed in a generic electrolyte 82;        contact areas 121, belonging to each of said dice 61 and, where applicable, to different segments belonging to each of said dice 61;        a counter-electrode 81;        a fixture 71′, containing a plurality of point contacts 66;        a voltage generator E having a first pole connected to said plurality of point contacts 66 and isolated from said electrolyte 82 by way of a sheath 24, and a second pole connected to said counter-electrode 81;        bi-directional arrows 84, indicating the direction of motion of the ions during deposition or removal;        ion depositing or removal zones 86; and        ion transit zones 87.        
Each point 66 is in electrical contact with one of the contact areas 121, and is contained in a dry volume 85′, kept separate from the electrolyte 82 by a seal 83′, shown in section view. The contact areas 121 are thus connected to one and the same potential.
The topology of the various layers and the design of the corresponding masks are highly complex: in this invention, what is proposed is a disposition of the equipotential connections that considerably simplifies topology of the layers and design of the masks, requiring a single contact area 121, a single point contact 66, a single dry volume 85 and a single seal 83, and permitting the use of a simplified fixture 71, as illustrated schematically in FIG. 12.